Carbon is a very important element. It is not the most abundant element in the universe or even on the Earth, but it is the second most common element in the human body. You could not live without carbon. If something you eat has protein or carbohydrates or fats, then it contains carbon. When your body breaks down that food to produce energy, you breathe out carbon dioxide. Carbon is also a very important element on Earth. Carbon is provided by the environment, moves through organisms and then returns to the environment again. When all this happens in balance, the ecosystem remains in balance too. In this section, let's follow the path of a carbon atom over many years and see what happens.

Nitrogen is also a very important element. Nitrogen must be converted to a useful form so that plants can grow. Without "fixed" nitrogen, plants and therefore animals could not exist as we know them.

The short term cycling of carbon begins with carbon dioxide and the process of photosynthesis. Our atmosphere is mostly made of nitrogen and oxygen, but there is a small amount of carbon dioxide in the air too. Plants and algae use this carbon dioxide, along with water and energy from sunlight to produce their own food. This is a little miracle that is happening everywhere around you each and every day. Plants and algae have the ability to take the inorganic carbon in carbon dioxide and make it into organic carbon, which is food. That is something that we cannot do at all! Imagine the difference between what would happen if you tried to eat a piece of coral or a shell and what happens when we eat sugar. We can't get energy from the bits of rock at all, but you know how quickly sugar can be used for energy in our bodies.

Through photosynthesis, carbon dioxide plus water and energy from sunlight is transformed into food with oxygen given off as a waste product. Chemists write shorthand equations for different types of chemical reactions. The equation for photosynthesis looks like this (Figure 18.14):

6CO2 + 6H2O + energy from sunlight → C6H12O6 + 6O2

Figure 18.14: Carbon dioxide, water, and energy from sunlight are turned into glucose (sugar) and oxygen through photosynthesis.

The amazing transformation that has happened here is changing energy from sunlight into chemical energy that plants and animals can use as food (Figure 18.15).

Figure 18.15: This diagram of the carbon cycle shows some of the places a carbon atom might be found. The black numbers indicate how much carbon is stored in various reservoirs, in billions of tons ("GtC" stands for gigatons of carbon; figures are circa 2004). The purple numbers indicate how much carbon moves between reservoirs each year. The sediments, as defined in this diagram, do not include the ∼70 million GtC of carbonate rock and kerogen.

As described above, an individual carbon atom could cycle very quickly if the plant takes in carbon dioxide to make food and then is eaten by an animal, which in turn breathes out carbon dioxide. Carbon might also be stored as chemical energy in the cells of the plant or the animal. If this happens, the carbon will stay stored as part of the organic material that makes up the plant or animal until it dies. Some of the time, when a plant or animal dies, it decomposes and the carbon is released back into the environment. Other times, the organic material of the organism is buried and transformed over millions of years into coal, oil, or natural gas. When this happens, it can take millions of years before the carbon becomes available again.

Another way that carbon is stored for long periods of time happens when carbon is used by ocean organisms. Many ocean creatures use calcium carbonate (CaCO3) to make their shells or to make the reef material where coral animals live. When algae die, their organic material becomes part of the ocean sediments, which may stay at the bottom of the ocean for many, many years. Over millions of years, those same ocean sediments can be forced down into the mantle when oceanic crust is consumed in deep ocean trenches. As the ocean sediments melt and form magma, carbon dioxide is eventually released when volcanoes erupt.

We can think of different areas of the ecosystem that use and give back carbon as carbon sources and carbon sinks. Carbon sources are places where carbon enters into the environment and is available to be used by organisms. One source of available carbon in the environment happens when an animal breathes out carbon dioxide. So carbon dioxide added to our atmosphere through the process of respiration is a carbon source. Carbon sinks are places where carbon is stored because more carbon dioxide is absorbed than is emitted. Healthy living forests and our oceans act as carbon sinks.

In the natural situation, the amount of carbon dioxide in the atmosphere is very low. This means that we can quickly change the amount of carbon dioxide in our atmosphere. Scientists can use data from air bubbles trapped in the ice of glaciers to determine what the natural level of carbon dioxide was before the Industrial Revolution, when humans began to use lots of fossil fuels. Measurements of the different gases in the air bubbles tell us that the natural level of carbon dioxide was about 280 parts per million. Today the amount of carbon dioxide in our atmosphere is 388 parts per million and that amount continues to rise every year. Scientists have been making measurements in the middle of the Pacific Ocean, far from any large land areas for fifty years. The graph (Figure 18.16) of this data shows that the amount of carbon dioxide has been steadily increasing every year.

Humans have changed the natural balance of the carbon cycle because we use coal, oil, and natural gas to supply our energy demands. Remember that in the natural cycle, the carbon that makes up coal, oil, and natural gas would be stored for millions of years. When we burn coal, oil, or natural gas, we release the stored carbon in the process of combustion. That means that combustion of fossil fuels is also a carbon source.

The equation for combustion of propane, which is a simple hydrocarbon looks like this (Figure 18.17):

C3H8 + 5O2 → 3CO2 + 4H2O

Figure 18.17: Combustion of propane involves propane and oxygen reacting to become carbon dioxide and water.

The equation shows that when propane burns, it uses oxygen and the result is carbon dioxide and water. So each time we burn a fossil fuel, we increase the amount of carbon dioxide in the atmosphere. Another way that carbon dioxide is being added to our atmosphere is through the cutting down of trees, called deforestation (Figure 18.18). Trees are very large plants, which naturally use carbon dioxide while they are alive. When we cut down trees, we lose their ability to absorb carbon dioxide and we also add the carbon that was stored in the tree into the environment. Healthy living forests act as a carbon sink, but when we cut them down, they are a carbon source.

Figure 18.18: This forest in Mexico has been cut down and burned to clear forested land for agriculture.

Coal, oil, and natural gas as well as calcium carbonate rocks and ocean sediments are long term carbon sinks for the natural cycling of carbon. When humans extract and use these resources, combustion makes them into carbon sources.

You may wonder why scientists study the carbon cycle or why we would be concerned about such small amounts of carbon dioxide in our atmosphere. Carbon dioxide is a greenhouse gas (Figure 18.19). Different gases in our atmosphere absorb infrared energy, the longer wavelengths of the Sun's reflected rays. These gases hold onto heat energy that would otherwise radiate out into space. As the heat is held in our atmosphere, it warms the Earth. This is just like what happens in a greenhouse. The glass that makes up the greenhouse holds in heat that would otherwise radiate out.

Figure 18.19: This diagram explains the role of greenhouse gases in our atmosphere.

When our atmosphere holds onto more heat than it would in the natural situation, it produces global warming. As our Earth continues to warm, there are many potential consequences. One possibility is that the current weather patterns will change. With rain falling in different areas, we won't be able to grow crops in the same regions which will impact our ability to grow food. Another possibility is that our polar ice caps will melt. We can already see this happening today. Glaciers all over the world are retreating as they melt away. Another possible consequence is that some species of plants and animals could become extinct. Polar bears have recently been added to the endangered species list as threatened because they need sea ice in order to hunt (Figure 18.20).

Figure 18.20: Polar bears depend on sea ice for hunting.

As continental glacial ice melts, this will cause sea levels to rise, which will cause flooding of low lying coastal areas. That would be a big problem because many of our biggest cities are along coastlines.

Nitrogen (N2) is also vital for life on Earth as an essential component of organic materials. Nitrogen is found in all amino acids, proteins, and nucleic acids such as DNA and RNA. Chlorophyll molecules in plants, which are used to create food by photosynthesis forming the basis of the food web, contain nitrogen.

Although nitrogen is the most abundant gas in the atmosphere, it is not in a form that plants can use. To be useful, nitrogen must be "fixed", or converted into a more useful form. Although some nitrogen is fixed by lightning or blue-green algae, much is modified by bacteria in the soil. These bacteria combine the nitrogen with oxygen or hydrogen to create nitrates or ammonia.

Nitrogen fixing bacteria either live free or in a symbiotic relationship with leguminous plants (peas, beans, peanuts). The symbiotic bacteria use carbohydrates from the plant to produce ammonia that is useful to the plant. Plants use this fixed nitrogen to build amino acids, nucleic acids (DNA, RNA) and chlorophyll. When these legumes die, the fixed nitrogen they contain fertilizes the soil.

Animals eat plant tissue and create animal tissue. After a plant or animal dies or an animal excretes waste, bacteria and some fungi in the soil fix the organic nitrogen and return it to the soil as ammonia. Nitrifying bacteria oxidize the ammonia to nitrites, other bacteria oxide the nitrites to nitrates, which can be used by the next generation of plants. In this way, nitrogen does not need to return to a gas. Under conditions when there is not oxygen, some bacteria can reduce nitrates to molecular nitrogen.

The Nitrogen Cycle

Usable nitrogen is sometimes the factor that limits how many organisms can grow in an ecosystem. Modern agricultural practices increase plant productivity adding nitrogen fertilizers to the soil. This can have unintended consequences as excess fertilizers run off the land, end up in water, and then cause nitrification of ponds, lakes and nearshore oceanic areas. Also, nitrogen from fertilizers may return to the atmosphere as nitrous oxide or ammonia, both of which have deleterious effects. Nitrous oxide contributes to the breakdown of the ozone layer and ammonia contributes to smog and acid rain.